Nonaqueous secondary battery

ABSTRACT

To provide a novel structure of a separator in a secondary battery. A nonaquesous secondary battery includes a positive electrode, a negative electrode, an electrolyte solution, a first separator, and a second separator. The first separator and the second separator are provided between the positive electrode and the negative electrode. The first separator is provided with a first pore, the second separator is provided with a second pore, and the size of the first pore is different from the size of the second pore. Furthermore, the proportion of the volume of the first pores in the first separator is different from the proportion of the volume of the second pores in the second separator.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the proem invention relates to a nonaqueous secondarybattery and a method for manufacturing the nonaqueous secondary battery.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, alight-emitting device, a power storage device, a storage device, amethod for driving any of them, and a method for manufacturing any ofthem.

2. Description of the Related Art

In recent years, a variety of power storage devices, for example,secondary batteries such as lithium-ion secondary batteries, lithium-ioncapacitors, air batteries, and fuel batteries have been activelydeveloped (Patent Documents 1 to 3). In particular, demand forlithium-ion secondary batteries with high output and high energy densityhas rapidly grown with the development of the semiconductor industry andwith the growth of demand for energy saving, for electrical devices, forexample, portable information terminals such as cellular phones,smartphones, and laptop personal computers, portable music players, anddigital cameras; medical equipment; next-generation clean energyvehicles such as hybrid electric vehicles (HEVs), electric vehicles(EVs), and plug-in hybrid electric vehicles (PHEVs); stationary powerstorage devices; and the like, the lithium-ion secondary batteries areessential for today's information society.

A lithium-ion secondary battery, which is one of nonaqueous secondarybatteries, includes a positive electrode, a negative electrode, aseparator, a nonaqueous electrolyte solution, and an exterior bodycovering these components. In lithium-ion secondary batteries, positiveelectrodes and negative electrodes are generally used; the positiveelectrodes each include a positive electrode current collector made ofaluminum or the like and a positive electrode mix which includes apositive electrode active material capable of occluding and releasinglithium ions and which is applied to both surfaces of the positiveelectrode current collector, and the negative electrodes each include anegative electrode current collector made of copper or the like and anegative electrode mix which includes a negative electrode activematerial capable of occluding and releasing lithium ions and which isapplied to both surfaces of the negative electrode current collector.These positive and negative electrodes are insulated from each other bya separator provided therebetween, and the positive electrode and thenegative electrode are electrically connected to a positive electrodeterminal and a negative electrode terminal, respectively, which areprovided on the exterior body. The exterior body has a certain shapesuch as a cylindrical shape or a rectangular shape.

REFERENCES Patent Documents

[Patent Document 1] PCT International Publication No. WO2012/16538

[Patent Document 2] United States Patent Application Publication No.2012/ 0002349

[Patent Document 3] PCT International Publication No. WO2009/131180

SUMMARY OF THE INVENTION

The separator is provided between the positive electrode and thenegative electrode and has a function of preventing direct contactbetween the electrodes. If the electrodes directly contact each other inthe lithium-ion secondary battery, an uncontrollable high current flowsbetween the electrodes, and, for example, a large amount of heat isgenerated, causing a safety hazard in some cases. Even when a safetyhazard is not caused, self-discharge occurs and a function as thebattery is impaired.

Furthermore, in a process of manufacturing or charging/discharging thelithium-ion secondary battery, a part of carrier ions contributing tocharging/discharging is deposited on a surface of the negative electrodeand becomes an irreversible component, which impairs a function as thebattery. When the deposition of lithium further proceeds, it becomes awhisker-like structure (whisker) and grows in some cases. The structuremight pass through a pore in the separator and connect the electrodesdepending on the property of the separator, which also causes a problem.

Furthermore, in order for the battery to function, the separator needsto have a function of holding the electrolyte solution. In addition,ionic conductivity is required for the separator Note that the ionicconductivity and the function of holding the electrolyte solution aresignificantly related to the property of the pore in a layer of theseparator.

The separator is generally formed using a porous material. The size andthe shape of a pore formed in the separator and the proportion of thevolume of pores in the layer of the separator (i.e., porosity) depend ona material of the separator. As the size of the pore in the separator islarger or the proportion of the volume of the pores (porosity) ishigher, ionic conductivity becomes higher whereas a function ofinsulating the electrodes is more impaired. In the case where awhisker-like structure (whisker) is generated, the whisker-likestructure penetrates through the separator more easily, which lowersinsulation performance. On the other hand, the pore in the separator isless likely to be blocked by a separated lump of the above-describedirreversible component formed through the deposition of lithium or othercomponents or by u particle of an active material separated from anactive material layer; accordingly, ionic conductivity is kept moreeasily.

The sizes and the amount of pores formed in the separator are determinedby a material and a formation method of the separator In order to obtaina separator having predetermined ionic conductivity and mechanicalstrength, the selection of the material and the formation method needsto be examined minutely. However, there is a limitation on the selectionof the material and the formation method of the separator; it is noteasy to form a separator having a desired property.

Furthermore, in a lithium-ion secondary battery having flexibility,various kinds of stress are generated inside the secondary battery inaccordance with change in the shape of the secondary battery. In thecase where the secondary battery docs not have a structure for relievingthe stress, shear failure occurs easily at a portion of the secondarybattery, so that a function as the secondary battery is lost.

In view of the above, an object of one embodiment of the presentinvention is to provide a separator having desired ionic conductivityand mechanical strength while preventing direct contact betweenelectrodes in a secondary battery. Another object is to achievelong-term reliability of a secondary battery.

Another object of one embodiment of the present invention is to providea novel structure of a separator in a lithium-ion secondary battery.Another object of one embodiment of the present invention is to providea novel power storage device or the like.

Another object of one embodiment of the present invention is to providea secondary battery that can be changed in shape, i.e., a secondarybattery having flexibility. Another object is to provide a novelseparator which can resist change in shape in a secondary battery havingflexibility.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can lie derived from the description of the specification, thedrawings, the claims, and the like.

A structure of one embodiment of the invention disclosed in thisspecification is a nonaqueous secondary battery including a positiveelectrode, a negative electrode, an electrolyte solution, a firstseparator, and a second separator. The first separator and the secondseparator are provided between the positive electrode and the negativeelectrode. The first separator is provided with a first pore, the secondseparator is provided with a second pore, and the size of the first poreis different front the size of the second pore.

Note that the thickness of the first separator may be different from thethickness of the second separator.

A structure of another embodiment of the invention disclosed in thisspecification is a nonaqueous secondary battery including a positiveelectrode, a negative electrode, an electrolyte solution, a firstseparator, a second separator, and a third separator. The firstseparator is provided between the positive electrode and the negativeelectrode, the second separator is provided between the first separatorand the positive electrode, and the third separator is provided betweenthe first separator and the negative electrode. The first separator isprovided with a first pore, the second separator is provided with asecond pore, the third separator is provided with a third pore, and thesize of the first pore is different from the size of the second pore andthe size of the third pore.

A structure of another embodiment of the invention disclosed in thisspecification is a nonaqueous secondary battery including a positiveelectrode, a negative electrode, an electrolyte solution, a firstseparator, a second separator, and a third separator. The firstseparator is provided between the positive electrode and the negativeelectrode, the second separator is provided between the first separatorand the positive electrode, and the third separator is provided betweenthe first separator and the negative electrode. The first separator isprovided with a first pore, the second separator is provided with asecond pore, the third separator is provided with a third pore, and thesize of the first pore is larger than the size of the second pore andthe size of the third pore.

A structure of another embodiment of the invention disclosed in thisspecification is a nonaqueous secondary battery including a positiveelectrode, a negative electrode, an electrolyte solution, a firstseparator, a second separator, and a third separator. The firstseparator is provided between the positive electrode and the negativeelectrode, the second separator is provided between the first separatorand the positive electrode, and the third separator is provided betweenthe first separator and the negative electrode. The first separator isprovided with a first pore, the second separator is provided with asecond pore, the third separator is provided with a third pore, and thefirst pore, the second pore, and the third pore are different in size.

In the nonaqueous secondary battery of the structure of one embodimentof the invention disclosed in this specification, the thickness of thefirst separator is preferably different from the thickness of the secondseparator and the thickness of the third separator, and more preferably,smaller than the thickness of the second separator and the thickness ofthe third separator.

In the nonaqueous secondary battery of the structure of one embodimentof the invention disclosed in this specification, the electrolytesolution may contain a lithium ion. The nonaqueous secondary battery ofthe structure of one embodiment of the invention disclosed in thisspecification may further include an exterior body having flexibilityand may have flexibility.

In a nonaqueous secondary battery, when separators with a multilayerstructure are provided between a positive electrode and a negativeelectrode, and the separators have pores with different sizes,characteristics of the whole multilayer structure are determined by aproperty and a thickness of each separator, the proportion of the volumeof pores per unit volume of each separator, and the like. A material, athickness, and the proportion of the volume of the pores per unit volumeof each separator included in the multilayer structure, and the like canbe selected within a predetermined range. By variously settingproperties of the separators, characteristics of the whole multilayerstructure as the separator can be set in detail.

For example, in the ease of providing a separator having a predeterminedthickness between the positive electrode and the negative electrode, astack of separators including a first separator which is thick, has alarge pore, and has a high proportion of the volume of pores per unitvolume and a second separator which is thin, has a small pore, and has alow proportion of the volume of pores per unit volume is provided. Theseparator with such a structure can have an excellent property ofholding an electrolyte solution owing to the second separator and havehigh ionic conductivity owing to the first separator.

When the separator having a predetermined thickness is formed of only amaterial of the first separator, the separator has high ionicconductivity while having low ability to prevent electrodes from beingconnected through a whisker-like lithium deposit (whisker). In addition,the mechanical strength of the separator is low. When the separatorhaving a predetermined thickness is formed of only a material of thesecond separator, the separator has low ionic conductivity, though theelectrodes can be prevented from being connected through thewhisker-like lithium deposit (whisker). In the case where a lump ofdeposit or the like is generated and the size of a pore in the separatoris small, the pore Ls easily blocked by the deposit or the like, whichfurther decreases ionic conductivity.

Because there is a limitation on a material to be used for the separatorand a properly of the separator material, it is difficult to form anecessary separator in some eases. In the case of using the multilayerseparator having different properties, characteristics of the separatorcan be selected from a wider range than in the ease of using theseparator formed of a single material.

In the case where a flexible secondary battery including the multilayerseparator is changed in shape, stress to be generated in the separatorcan be relieved by the occurrence of sliding between the layers. Thus,the secondary battery including the multilayer separator has highresistance to change in shape. The structure including the multilayerseparator is suitably used for the flexible secondary battery.

Furthermore, when the flexible secondary battery is changed in shape ina state where a whisker-like structure (whisker) that has grown from thenegative electrode and penetrated through the interface between the twoseparators exists continuously in the two separators in the flexiblesecondary battery, the whisker-like structure (whisker) cannot enduresliding that occurs between the two separators because of change in theshape of the secondary battery, and thus, the whisker-like structure(whisker) is broken by shear at the interface between the separators. Inthis manner, connection (short-circuit) between the electrodes due tothe whisker-like structure (whisker) can be prevented.

lire exterior body of the flexible secondary battery of one embodimentof the present invention can be changed in shape in the range of radiusof curvature of 10 mm or more, preferably 30 mm or more. The exteriorbody of the secondary battery is formed of one film or two films. In thecase where the secondary battery has a layered structure, across-sectional structure of the battery that is bent is surrounded bytwo curves of the film serving as the exterior body.

Description is given of the radius of curvature of a surface withreference to FIGS. 17A to 17C. In FIG. 17A, on a plane 1701 along whicha curved surface 1700 is cut, part of a curve 1702, which is a form ofthe curved surface, is approximate to an are of a circle, and the radiusof the circle is referred to us a radius 1703 of curvature and thecenter of the circle is referred to as a center 170-4 of curvature. FIG.17B is a top view of the curved surface 1700. FIG. 17C is across-sectional view of the curved surface 1700 taken along the plane1701. When a curved surface is cut along a plane, the radius ofcurvature of a curve, which is a form of the curved surface, depends onplane along which the curved surface is cut Here, the radius ofcurvature of a curved surface is defined as the radius of curvature of acurve, which is a cross-sectional form of the curved surface, on a planealong which the cursed surface is cut such that the curve has thesmallest radius of curvature.

In the case of curving a secondary battery in which a component 1805including electrodes and an electrolyte solution is sandwiched betweentwo films as exterior bodies, a radius 3802 of curvature of a film 1801close to a center 1800 of curvature of the secondary battery is smallerthan a radius 1804 of curvature of a film 1803 far from the center 1800of curvature (FIG. 18A). When the secondary battery is curved and has anarc-shaped cross section, compressive stress is applied to a surface ofthe film close to the center 3800 of curvature and tensile stress isapplied to a surface of the film far from the center 1800 of curvature(FIG. 18B). However, by forming a pattern of projections and depressionson surfaces of the exterior bodies, influence of distortion can bereduced to be acceptable even when the compressive stress and thetensile stress are applied For this reason, the secondary battery canchange its form such that the exterior body on the side closer to thecenter of curvature has a curvature radius greater than or equal to 10mm, preferably greater than or equal to 30 mm.

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partiallyarc-shaped; for example, a shape illustrated in FIG. 18C, a wavy shapeillustrated in FIG. 18D, and an S shape can be used. When the curvedsurface of the secondary battery has a shape with a plurality of centersof curvature, the secondary battery can change its form such that acurved surface with the smallest radius of curvature among radii ofcurvature with respect to the plurality of centers of curvature, whichis a surface of the exterior body on the side closer to the center ofcurvature, has a curvature radius greater than or equal to 10 mm,preferably greater than or equal to 30 mm.

One embodiment of the present invention can provide a separator havingdesired ionic conductivity and mechanical strength while preventingdirect contact between electrodes in a secondary battery. One embodimentof the present invention can provide can achieve long-term reliabilityof a secondary battery.

One embodiment of the present invention can provide a novel structure ofa separator in a lithium-ion secondary battery. One embodiment of thepresent invention can provide a novel power storage device or the like.

One embodiment of the present invention can provide a secondary batterythat can be changed in shape, i.e., a secondary battery havingflexibility. One embodiment of the present invention can provide a novelseparator which can resist change in shape in a secondary battery havingflexibility.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are cross-sectional schematic views showing oneembodiment of the present invention.

FIGS. 2A and 2B are cross-sectional schematic views showing shearing ofa whisker-like structure.

FIGS. 3A and 3B are cross-sectional schematic views showing a currentcollector and an active material.

FIGS. 4A to 4D are cross-sectional schematic views showing oneembodiment of the present invention.

FIGS. 5A to 5C illustrate a coin-type secondary battery.

FIG. 6 illustrates a secondary battery having a stacked-layer structure.

FIGS. 7A and 7B illustrate a cylindrical secondary battery.

FIGS. 8A and 8B illustrate an example of a power storage device.

FIGS. 9A1, 9A2, 9B1, and 9B2 illustrate examples of a power storagedevice.

FIGS. 10A and 10B illustrate examples of a power storage device.

FIGS. 11A and 11B illustrate examples of a power storage device.

FIG. 12 illustrates an example of a power storage device.

FIGS. 13A to 13F illustrate electronic devices.

FIGS. 14A to 14C illustrate an electronic device.

FIGS. 15A and 15B are cross-sectional schematic views showing oneembodiment of the present invention.

FIGS. 16A and 16B are cross-sectional schematic views showing oneembodiment of the present invention.

FIGS. 17A to 17C illustrate the rad us of curvature.

FIGS. 18A to 18D illustrate a secondary battery having flexibility.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. However, the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. Further, the present invention is notconstrued as being limited to description of the embodiments.

Note that in each drawing described in this specification, the size ofeach component, such as the thickness and the size of a positiveelectrode, a negative electrode, an active material layer, an exteriorbody, and the like is exaggerated for clarity in some cases. Therefore,the sizes of the components are not limited to the sizes in the drawingsand relative sizes between the components.

Ordinal numbers such as “first”, “second”, and “third” are used forconvenience and do not denote the order of steps or the stacking orderof layers. Therefore, for example, description can be made even when“first” is replaced with “second”, “third”, or the like as appropriate.In addition, the ordinal numbers in this specification and the like arenot necessarily the same as those which specify one embodiment of thepresent invention.

Note that in the structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions in different drawings are denoted by the same referencenumerals, and description of such portions is not repeated. Further, thesame hatching pattern is applied to portions having similar functions,and the portions are not especially denoted by reference numerals insome cases.

in this specification and the like, the phrase “pore in a separator”refers to a region without a separator material in a layer or a filmincluding the separator material. The term “separator” refers to aseparator provided with a pore or refers to a separator material whichis distinguished from a pore. There is no particular limitation onwhether the shape of fixe pore is a shape having a correlation with agiven direction or an isotropic shape. There is no limitation on whetheror not one pore penetrates through a layer including the separatormaterial and whether or not one pore is a closed space surrounded by theseparator material. The phrase “pore in a separator” also refers to apore which is wholly or partly filled with an electrolyte solution in asecondary battery.

The descriptions in embodiments for the present invention can becombined with each other as appropriate.

Embodiment 1

Description is given below of a method for manufacturing a lithium-ionsecondary battery of one embodiment of the present invention withreference to FIG. 1A first, a cross-sect usual schematic view of a stackof a positive electrode current collector 110, a positive electrodeactive material layer 112, a first separator 100, a second separator101, a negative electrode active material layer 113, and a negativeelectrode current collector 111 is shown. Details of the currentcollector and the active material layer are described later. Note thatthe active material layer can be formed on both surfaces of the currentcollector, und the secondary battery can have a stacked-layer structure.

FIGS. 1B and 1C are enlarged cross-sectional schematic views of thefirst Separator 100 and the second separator 101, respectively The firstseparator 100 and the second separator 101 have a first pore 104 and asecond pore 105, respectively. The size of the first pore 104 isdifferent from that of the second pore 105. The proportion of the volumeof the first pores 104 in the first separator 100 is different from theproportion of the volume of the second pores 105 in the second separator101. Note that in the schematic views, each separator has a fibrousstructure and the pore is shown as an interstice between fibers;however, the structure of the separator is not limited to a fibrousstructure.

As shown in FIG. 1A, the first separator 100 and the second separator101 have different thicknesses. Therefore, the first separator 100 andthe second separator 101 are different from each other incharacteristics as a separator, such as ionic conductivity, mechanicalstrength, and insulation performance. Note that even when the firstseparator 100 and the second separator 101 are formed using the samematerial, different characteristics may be provided for the separatorsby changing a formation method, formation conditions, or the like.

The separator may be formed using a material such as paper, nonwovenfabric, a glass fiber, a synthetic fiber such as nylon (polyimide),vinylon (a polyvinyl alcohol based fiber), polyester, acrylic,polyolefin, or polyurethane. However, a material which does not dissolvein an electrolyte solution described later should be selected.

More specifically, as a material for the separator, high-molecularcompounds based on fluorine-based polymer, polyether such aspolyethylene oxide and polypropylene oxide, polyolefin such aspolyethylene and polypropylene, polyacrylonitrile, polyvinylidenechloride, polymethyl methacrylate, polymethylacrylate, polyvinylalcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,polyethyleneimine, polybutadiene, polystyrene, polyisoprene, andpolyurethane, derivatives thereof, cellulose, paper, nonwoven fabric,and a glass fiber can be used either alone or in combination.

The separator needs to have insulation performance that preventsconnection between the electrodes, performance that holds theelectrolyte solution, and ionic conductivity. As a method for forming afilm having a function as a separator, a method for forming a film bystretching is given. Examples of the method include a Stretchingaperture method in which a melted polymer material is spread, heat isreleased from the material, and pores are formed by stretching theresulting film in the directions of two axes parallel to the film. Notethat the range of the thickness of the film and the size of the pore inthe film of the separator obtained by stretching is limited by amaterial of the separator and mechanical strength of the film.

In this embodiment, the first separator 100 and the second separator 101in the secondary battery can be formed by a stretching method. The firstseparator 100 and the second separator 101 can be formed using one ormore kinds of material selected from the above-described materials ormaterials other than those described above. Characteristics such as thesize of the pore in the film, the proportion of the volume of the poresin the film (also referred to as porosity), and the thickness of thefilm can be determined by film formation conditions, film stretchingconditions, and the like. By using the first separator 100 and thesecond separator 101 having different characteristics, the performanceof the separators of the secondary battery can be selected morevariously than in the case of using one of the separators.

For example, in the secondary battery including the first separator 100and the second separator 101 as the separator, the second separator 101is formed using a film in which the size of the pore is smaller, theproportion of the volume of the pores in the film (porosity) is lower,and the thickness of the film is smaller than those of the firstseparator 100. In that case, the second separator 101 can obtaininsulation performance that prevents, for example, connection between apositive electrode and a negative electrode through a whisker-like Listructure (whisker), whereas the first separator 100 having a largethickness can obtain high ionic conductivity. That is, the firstseparator 100 and the second separator 101 can have different functions.

The structure including the two separators is also suitable as astructure of a separator in a flexible secondary battery. In the casewhere flow stress is applied to the secondary battery, the stress can berelieved by sliding of the first separator 100 and the second separator101 at the interface between the separators. Furthermore, in the casewhere the flexible secondary battery includes a whisker-like structure(whisker) 108 that has grown from the negative electrode and penetratedthrough the interface between the separators (FIG. 2A), the whisker-likestructure (whisker) 108 cannot endure sliding that occurs between thetwo separators because of change in the shape of the secondary battery,and thus, the whisker-like structure (whisker) 108 is broken by shear atthe interface between the separators. In this manner, connection(short-circuit) between the electrodes due to the whisker-like structure(whisker) 108 can be prevented (FIG. 2B). An outline arrow shown in FIG.2B indicates the sliding of each separator.

As a method for forming a structure including the two separators, thereis a method in which two layers each including a film containing aseparator material are stretched to form pores in both of the layers. Inthis method, the two layers are Stretched under the same conditions:therefore, the sizes of the pores formed in the separators may besimilar to each other and the proportions of the volume of the pores(porosity) in the separators may be similar to each other, therefore, insome eases, it is difficult to obtain the separators different inperformance. Note that one embodiment of the present invention docs notnecessarily exclude a stack of separators formed by the method.

As a method for incorporating the two separators into a secondarybattery, after the two separators are stacked, the separators can beprovided between the positive electrode and the negative electrode whichare to be described later. Alternatively, after one separator is placedon each of the positive electrode and the negative electrode, theelectrodes can be stacked. Further alternatively, after a firstseparator and a second separator is placed on one of the positiveelectrode and the negative electrode, the other of the positiveelectrode and the negative electrode can be stacked thereon. Across-sectional schematic view of the secondary battery including thetwo separators is shown in FIG. 1A.

In the case where the second separator 101 is formed into a sheet-likeshape large enough to cover both surfaces of the negative electrode orinto an envelope-like shape to envelop the negative electrode, thenegative electrode can be protected from mechanical damage and can beeasily handled in manufacturing the secondary battery. The firstseparator 100 is provided between the negative electrode enveloped bythe second separator 101 and the positive electrode active materiallayer 112, the resulting component is stored in the exterior body 120,and the exterior body 120 is filled with the electrolyte solution 121.In this manner, the secondary battery can be manufactured. FIGS. 15A and15B show cross-sectional structures of secondary batteries eachincluding the second separator 101 having an envelope-like shape. FIG.15A shows a cross-sectional structure of the secondary battery includinga pair of the positive electrode and the negative electrode. A secondarybattery having a stacked-layer structure including a plurality of pairsof the positive electrode and the negative electrode can bemanufactured. FIG. 15B shows a cross-sectional structure of thesecondary battery having a stacked-layer structure. In the secondarybattery having a stacked-layer structure, the electrode active materialis formed on both surfaces of the current collector, and the resultingcomponent is enveloped by an envelope-like separator.

The first separator and the second separator slide against each other ina flexible secondary battery in some eases as described below; in viewof this, the first separator and the second separator are not fixed.

The negative electrode is described with reference to FIG. 3A. Thenegative electrode includes at least the negative centrode activematerial layer 113 and the negative electrode current collector 111. Inthis embodiment, steps of forming the negative electrode with the use ofa carbon-based material as a material for the negative electrode activematerial layer 113 are described below. Note that in FIG. 3A, thenegative electrode active material is in the form of particles. For thisreason, the negative electrode active material is schematicallyillustrated as circles in FIG. 3A; however, the shape of the negativeelectrode active material is not limited to this shape. Furthermore,although the particles of the negative electrode active material havingseveral sizes are schematically shown, the sizes may vary. Steps offorming the negative electrode are described below.

Examples of the carbon-based material as the negative electrode activematerial include graphite, graphitizing carbon (soft carbon),non-graphitizing carbon (hard carbon), a carbon nanotube, graphene, andcarbon black. Examples of the graphite include artificial graphite suchas meso-carbon microbeads (MCMB), coke-based artificial graphite, orpitch-based artificial graphite and natural graphite such as sphericalnatural graphite In addition, the shape of the graphite is a flaky shapeor a spherical shape, for example.

Other than the carbon-based material, a material which enablescharge-discharge reaction by alloying and dealloying reaction withlithium can be used as the negative electrode active material, forexample, a material including at least one of Ga, Si, Al, Ge, Sn, Pb,Sb, Bi, Ag, Zn, Cd, In, and the like can be used. Such elements havehigher capacity than carbon. In particular, silicon is preferablebecause of high theoretical capacity of 4200 mAh/g. Examples of analloy-based material using such elements include Mg₂Si, Mg₂Ge, Mg₂Sn,SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₆, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃,LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like.

Alternatively, us the negative electrode active material, oxide such asSiO, SnO, SnO₂, titanium dioxide (TiO₂), lithium titanium oxide(Li₄Ti₅O₁₂), lithium-graphite intercalation compound (Li_(x)C₆), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), molybdenum oxide (MoO₂), or thelike can be used.

Further alternatively, as the negative electrode active material,Li_(3−x)M_(x)N (M is Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedas the negative electrode active material. For example, a transitionmetal oxide with which an alloying reaction with lithium is not caused,such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), maybe used for the negative electrode active material. Other examples ofthe material which causes a conversion reaction include oxides such asFe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides such as CoS_(0.89), NiS, orCuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄, phosphides such as NiP₂,FeP₂, and CoP₃, and fluorides such as FeF₃ and BiF₃.

The particle diameter of the negative electrode active material ispreferably greater than or equal to 50 nm and less than or equal to 100μm, for example.

Examples of a conductive additive of the electrode include acetyleneblack (AB), graphite (black lead) particles, carbon nanotubes, graphene,and fullerene.

A network for electron conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the particles of the negativeelectrode active material. The addition of the conductive additive tothe negative electrode active material layer increases the electronconductivity of the negative electrode active material layer 113.

As a binder, instead of polyvinylidene fluoride (PVDF) as a typical one.polyimide, polytetrufluoroethylene, polyvinyl chloride,ethylene-propylene-diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose, or the like canbe used.

The content of the binder in the negative electrode active materiallayer 113 is preferably greater than or equal to 1 wt % and less than orequal to 10 wt %, more preferably greater than or equal to 2 wt % andless than or equal to 8 wt %, and still more preferably greater than orequal to 3 wt % and less than or equal to 5 wt %. The content of theconductive additive in the negative electrode active material layer 113is preferably greater than or equal to 1 wt % and less than or equal to10 wt %, more preferably greater than or equal to 1 wt % and less thanor equal to 5 wt %.

Next, the negative electrode active material layer 113 is formed on thenegative electrode current collector 111. In the case where the negativeelectrode active material layer 113 is formed by a coating method, thenegative electrode active material, the binder, the conductive additive,and a dispersion medium are mixed to form an electrode paste (slurry),and the electrode paste is applied to the negative electrode currentcollector 111 and dried. If necessary, pressing may be performed afterthe drying.

In this embodiment, copper foil is used as the negative electrodecurrent collector 111, and a mixture of MCMB and PVDF as the binder isused as the slurry.

The negative electrode current collector 111 can be formed using amaterial, which has high conductivity and is not alloyed with carrierions of lithium or the like, such as stainless steel, gold, platinum,zinc, iron, copper, titanium, tantalum, or an alloy thereof. Stillalternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal clement which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The negative electrode current collector 111 can have afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a cylindrical shape, a coil shape, a punching-metal shape, anexpanded-metal shape, or the like as appropriate. The negative electrodecurrent collector 111 preferably has a thickness greater than or equalto 5 μm and less than or equal to 30 μm. A part of the surface of theelectrode current collector may be provided with an undercoat layerusing graphite or the like.

Through the above steps, the negative electrode of the lithium-ionsecondary battery can be formed.

The positive electrode is described. The positive electrode includes atleast the positive electrode active material layer 112 and the positiveelectrode current collector 110. In this embodiment, steps of formingthe positive electrode with the use of lithium iron phosphate (LiFePO₄)as the positive electrode active material layer 112 are described below.Note that in FIG. 3B, the positive electrode active material is in theform of panicles For this reason, the positive electrode active materialis schematically illustrated as circles in FIG. 3B; however, the shapeof the positive electrode active material is not limited to this shape.Although the particles of the positive electrode active material havingseveral uniform sizes are schematically shown, the sizes may vary. Stepsof forming the positive electrode are described below.

As the positive electrode active material, a material into/from whichcarrier ions such as lithium ions can be inserted and extracted is used,and examples of the material include a lithium-containing materialhaving an olivine crystal structure, a layered rock-salt crystalstructure, or a spinel crystal structure.

Typical examples of the lithium-containing material with an olivinecrystal structure (general formula: LiMPO₄ (M is Fe(II), Mn(II), Co(II),or Ni(II))) include LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPo₄,LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 021 c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

LiFePO₄ is particularly preferable because it properly satisfiesconditions necessary for the positive electrode active material, such assafety, stability, high capacity density, high potential, and theexistence of lithium ions which can be extracted in initial oxidation(charging).

Examples of the lithium-containing material with a layered rock-saltcrystal Structure include lithium cobalt oxide (LiCoO₂); LiNiO₃; LiMnO₂;Li₂MNO₃; an NiCo-based lithium-containing material (a general formulathereof is LiNi_(x)Co_(1−x)O₂ (0<x<1)) such as or LiNi_(0.8)Co_(0.2)O₂;an NiMn-based lithium-containing material (a general formula thereof isLiNi_(x)Mn_(1−x)O₂ (0<x<1)) such as LiNi_(0.5)Mn_(0.5)O₂; and anNiMnCo-based lithium-containing material (also referred to as NMC, and ageneral formula thereof is LiNi_(x)Mn_(y)Co_(1−x−y)O₂ (x>0, y>0, x+y<1)such as LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂. Moreover,Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, Li₂MnO₃-LiMO₂ (M=Co, Ni or Mn), andthe like can be given as the examples.

LiCoO₂ is particularly preferable because it has high capacity,stability in the air higher than that of LiNiO₂, and thermal stabilityhigher than that of LiNiO₂, for example.

Examples of the lithium-containing material with a spinel crystalstructure include LiMn₂O₄, Li_(1+x)Mn_(2−y)O₄, Li(MnA)₂O₄, andLiMn_(1.5)Ni_(0.5)O₄.

It is preferable to add a small amount of lithium nickel oxide (LiNiOorLiNi_(1−x)MO₂ (M=Co, Al, or the like)) to a lithium-containing materialwith a spinel crystal structure which contains manganese such as LiMn₂O₄because advantages such as minimization of the elution of manganese andthe decomposition of an electrolyte solution can be obtained.

Alternatively, as the positive electrode active material, a compositeoxide represented by a general formula Li(_(2−j))MSiO₄ (M is Fe(II),Mn(II), Co(II), or Ni(II), 0≤j≤2) can be used. Typical examples ofLi(_(2−j))MSiO₄ (general formula) are Li(_(2−j))FeSiO₄,Li(_(2−j))NiSiO₄, Li(_(2−j))CoSiO₄, Li(_(2−j))MnSiO₄,Li(_(2−j))Fe_(k)Ni_(l)SiO₄, Li(_(2−j))Fe_(k)Co_(l)SiO₄,Li(_(2−j))Fe_(k)Mn_(i)SiO₄, Li(_(2−j))Ni_(k)Co_(l)SiO₄,Li(_(2−j))Ni_(k)Mn_(l)SiO₄ (k+≤1, 0<k< 1, and 0<l<1),Li(_(2−j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li(_(2−j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li(_(2−j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi(_(2−j))Fe_(r)Ni_(s)Co_(t)Mn_(a)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb. or Al, X=S, P,Mo, W, As, or Si) can be used as the positive electrode active material.Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, andLi₃Fe₂(PO₄)₃. Still further alternatively, a compound represented by ageneral formula, Li₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (M=Fe or Mn), aperovskite fluoride such as NaF₃ or FeF₃, a metal chalcogenide (asulfide, a selenite, or a telluride) such as TiS₂ or MoS₂, alithium-containing material with an inverse spinel crystal structuresuch as LiMVO₄, a vanadium oxide-based (e.g., V₂O₅, V₆O₁₃, or LiV₃O₈), amanganese oxide, or an organic sulfur compound can be used as thepositive electrode active material, for example.

In the case where carrier ions are alkali metal ions other than lithiumions or alkaline-earth metal ions, the following may be used as thepositive electrode active material: a compound or oxide which isobtained by substituting an alkali metal (e.g., sodium or potassium) oran alkaline-earth metal (e.g., calcium, strontium, barium, beryllium, ormagnesium) for lithium in any of the above-described compounds oroxides. For example, the positive electrode active material may be alayered oxide containing sodium such as NaFcO₂ orNa_(2/3)[Fe_(1/2)Mn_(1/2)]O₂.

Further alternatively, any of the aforementioned materials may becombined to be used as the positive electrode active material. Forexample, a solid solution containing any of the aforementionedmaterials, e.g., a solid solution containingLiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ and Li₂MnO₃ may be used.

The average particle diameter of the primary particle of the positiveelectrode active material is preferably greater than or equal to 50 nmand less than or equal to 100 μm.

Examples of the conductive additive of the electrode include acetyleneblack CAB), graphite (black lead) particles, carbon nanotubes, graphene,and fullerene.

A network for electron conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the panicles of the positiveelectrode active material. The addition of the conductive additive tothe positive electrode active material layer increases the electronconductivity of the positive electrode active material layer 112.

As the binder, instead of PVDF as a typical one, polyimide.polytetrafluoroethylene, polyvinyl chloride, ethylene-propylene-dienepolymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber,fluorine rubber, polyvinyl acetate, polymethyl methacrylate,polyethylene, nitrocellulose, or the like can be used.

The content of the binder in the positive electrode active materiallayer 112 is preferably greater titan or equal to 1 wt % and less thanor equal to 10 wt %, more preferably greater than or equal to 2 wt % andless than or equal to 8 wt %, and still more preferably greater than orequal to 3 wt % and less than or equal to 5 wt %. The content of theconductive additive in the positive electrode active material layer 112is preferably greater than or equal to 1 wt % and less than or equal to10 wt %, more preferably greater than or equal to 1 wt % and less thanor equal to 5 wt %.

In the case where the positive electrode active material layer 112 isformed by a coating method, the positive electrode active material, thebinder, the conductive additive, and the dispersion medium are mixed toform an electrode paste (slurry), and the electrode paste may be appliedto the positive electrode current collector 110 and dried. In thisembodiment, a metal material including aluminum as its main component ispreferably used as the negative electrode current collector 111.

The positive electrode Current collector can be formed using a material,which has high conductivity and is not alloyed with carrier ions oflithium or the like, such as stainless steel, gold, platinum, aluminum,or titanium, or an alloy thereof. Alternatively, an aluminum alloy towhich an element which improves heat resistance. Such as silicon,titanium, neodymium, scandium, or molybdenum, is added can be used.Still alternatively, a metal clement which forms silicide by reactingwith silicon can be used. Examples of the metal element which formssilicide by reacting with silicon include zirconium, titanium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt,nickel, and the like. The positive electrode current collector can havea foil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate.

Through the above steps, the positive electrode of the lithium-ionsecondary battery can be formed.

A thin lithium-ion secondary battery can be manufactured using the firstseparator, the second separator, the positive electrode, the negativeelectrode, and the electrolyte solution which are obtained in thisembodiment. The first separator, the second separator, the positiveelectrode, and the negative electrode are incorporated in the secondarybattery as described above.

The electrolyte solution used in the lithium-ion secondary battery ispreferably a nonaqueous solution (solvent) containing an electrolyte(solute).

As a solvent for the electrolyte solution, an aptotic organic solvent ispreferably used. For example, one of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate, chloroethylene carbonate, vinylenecarbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate,methyl acetate, methyl butyrate, 1,3-dioxane, 1.4-dioxane,dimethoxyethane (DME) dimethyl sulfoxide, diethyl ether, methyl diglyme,acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone canbe used or two or more of these solvents can be used in an appropriatecombination in an appropriate ratio.

When a gelled polymer material is used as the solvent of the electrolytesolution, safety against liquid leakage and the like is improved.Further, the lithium-ion secondary battery can be thinner and morelightweight. Typical examples of the gelled high-molecular materialinclude a silicone gel, an acrylic gel, an acrylnitrile gel,polyethylene oxide, polypropylene oxide, a fluorine-based polymer, andthe like.

Alternatively, the use of one or more of ionic liquids (room temperaturemolten salts) that have non-flammability and non-volatility as thesolvent for the electrolyte solution can prevent a lithium-ion secondarybattery from exploding or catching fire even when the lithium-ionsecondary battery internally shorts out or the internal temperatureincreases due to overcharging or the like. Thus, the lithium-ionsecondary battery has improved safety.

Examples of an electrolyte dissolved in the above-described solvent areone of lithium salts such as LiPF₆, LiClO₄, LiAsF₄, LiAlCl₄, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), andLiN(C₂F₅SO₂)₂, or two or more of these lithium salts in an appropriatecombination in an appropriate ratio.

Although the ease where carrier ions are lithium ions in the aboveelectrolyte is described, carrier ions other than lithium ions can beused. When the carrier ions other than lithium ions are alkali metalions or alkaline-earth metal ions, instead of lithium in the lithiumsalts, an alkali metal (e.g., sodium or potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, or magnesium) may beused for the electrolyte.

The electrolyte solution used for the secondary battery preferablycontains a small amount of dust particles and elements other than theconstituent elements of the electrolyte solution (hereinafter, alsosimply referred to as impurities) so as to be highly purified.Specifically, the weight ratio of impurities to the electrolyte solutionis less than or equal to 1%, preferably less than or equal to 0.1%, andmore preferably less than or equal to 0.01%. An additive agent such asvinylene carbonate may be added to the electrolyte solution.

When the positive electrode, the negative electrode, the firstseparator, and the second separator in this embodiment are each formedusing a flexible material, the secondary battery can have flexibility.In the flexible secondary battery, when flow stress is applied to thesecondary battery, the first separator 100 and the second separator 101are shifted from each other at the interface between the separators,whereby the stress can be relieved.

Note that one embodiment of the present invention can be used forvarious power storage devices. Examples of such a power storage deviceinclude a battery, a primary battery, a secondary battery, a lithium-ionsecondary battery, a lithium air battery, a lead storage battery, alithium-ion polymer secondary battery, a nickel-hydrogen storagebattery, a nickel-cadmium storage battery, a nickel-iron storagebattery, a nickel-zinc storage battery, a silver oxide-zinc storagebattery, a solid-state battery, and an air battery. In addition, acapacitor is given as another example of the power storage devices. Forexample, with a combination of the negative electrode of one embodimentof the present invention and an electric double layer positiveelectrode, a capacitor such as a lithium ion capacitor can bemanufactured.

Note that this embodiment can be implemented in free combination withany of the other embodiments.

Embodiment 2

In this embodiment, a method for manufacturing a lithium-ion secondarybattery of one embodiment of the present invention is described belowwith reference to FIG. 4A. First, a cross-sectional schematic view of astack of the positive electrode current collector 110, the positiveelectrode active material layer 112, the second separator 101, the firstseparator 100, a third separator 102, the negative electrode activematerial layer 113, and the negative electrode current collector 111 isshown. Note that the active material layer can be formed on bothsurfaces of the current collector, and the secondary battery can have astacked-layer structure.

FIGS. 4B. 4C, and 4D are enlarged cross-sectional schematic views of thesecond separator 101, the first separator 100, and the third separator102, respectively. The first separator 100, the second separator 101,and the third separator 102 have the first pore 104, the second pore105, and a third pore 106, respectively. Note that in the schematicviews, each separator has a fibrous structure and the pore is shown asan interstice between fibers; however, the structure of the separator isnot limited to a fibrous structure. The proportion of the volume of thefirst pores 104 in the first separator 100, the proportion of the volumeof the second pores 105 in the second separator 101, and the proportionof the volume of the third pores 106 in the third separator 102 can beconsidered to be elements contributing to the characteristics of theseparators. The characteristics as the separator, such as ionicconductivity and mechanical strength, can be determined by the size ofthe pore and the proportion of the volume of the pores (porosity) ineach separator.

Each separator can be formed using the material and the method describedin Embodiment 1. For example, in the case where the second separator 101and the third separator 102 having the same thickness are formed usingthe same material and the same method whereas the first separator 100 isformed using a material or a method which is different from that of thesecond separator 101 and the third separator 102, the first pore 104 inthe first separator 100 can be made larger than the second pore 105 inthe second separator 101, the proportion of the volume of the firstpores 104 in the first separator 100 can be made higher than theproportion of the volume of the second pores 105 in the second separator101, and the first separator can be made thicker than the secondseparator.

Insulation performance that prevents the positive electrode and thenegative electrode from being connected to each other can be obtainedmainly by the second separator 101 and the third separator 102, and highionic conductivity can be obtained by the first separator 100 having alarge thickness. In the case where the second separator 101 and thethird separator 102 are each formed into a sheet-like shape large enoughto cover both surfaces of the electrode or into an envelope-like shapeto envelop the electrode, the positive electrode and the negativeelectrode can be protected from mechanical damage and the positiveelectrode and the negative electrode can be easily handled inmanufacturing the secondary battery. The first separator 100 is providedbetween the positive electrode enveloped by the second separator 101 andthe negative electrode enveloped by the third separator 102, theresulting component is stored in the exterior body 120, and the exteriorbody 10 is filled with the electrolyte solution 121. In this manner, thesecondary battery can be manufactured FIGS. 16A and 16B showcross-sectional structures of secondary batteries each including thesecond separator 101 and the third separator 102 having an envelope-likeshape. FIG. 16A shows a cross-sectional structure of the secondarybattery including a pair of the positive electrode and the negativeelectrode. A secondary battery having a stacked-layer structureincluding a plurality of pairs of the positive electrode and thenegative electrode can be manufactured. FIG. 16B shows a cross-sectionalstructure of the secondary battery having a stacked-layer structure. Inthe secondary battery having a stacked-layer structure, the electrodeactive material is formed on both surfaces of the current collector, andthe resulting component is enveloped by an envelope-like separator.

Embodiment 1 can be referred to for materials of the positive electrode,the positive electrode active material, the negative electrode, thenegative electrode active material, and the electrolyte solution, and amethod for forming each electrode

When the separator between the positive electrode and the negativeelectrode bus a three-layer structure, stress generated by change in theshape of the flexible secondary battery can be relieved. That is twofilms can be shifted from each other at the interface between the firstseparator 100 and the second separator 101 and the interface between thefirst separator 100 and the third separator 102, which relieves stressdue to the change in shape. Thus, shear failure caused by the change inshape can be prevented in the secondary battery.

Note that a part or all of the materials, the thicknesses, and othercharacteristics of the second separator 101 and the third separator 102can be replaced with a part or all of the material, the thickness, andother characteristics of the first separator 100. Furthermore, thematerial, the thickness, the size of the pore, the proportion of thevolume of the pores in the film, and other characteristics of the secondseparator 101 can be partly or wholly different from the material, thethickness, the size of the pore, the proportion of the volume of thepores in the flint, and other characteristics of the third separator102. By using the three separators, the properties of the separator inthe secondary battery can be set variously. Furthermore, the separatorcan have a structure of four or more layers.

Embodiment 3

In this embodiment, structures of a secondary battery including theseparator described in Embodiment 1 or Embodiment 2 are described withreference to FIGS. 5A to 5C, FIG. 6, and FIGS. 7A and 7B.

(Coin-Type Secondary Battery)

FIG. 5A is an external view of u coin-type (single-layer flat type)secondary battery. FIG. 5B is a cross-sectional view of the coin-typesecondary battery.

In a coin-type secondary battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and scaled by a gasket 303 made of, for example, polypropylene. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308. A separator 310 a, aseparator 310 b, and an electrolyte (not illustrated) are includedbetween the positive electrode active material layer 306 and thenegative electrode active material layer 309.

For the positive electrode can 301 and the negative electrode can 302, ametal having corrosion resistance to an electrolyte solution, such asnickel, aluminum, or titanium, an alloy of such a metal, and an alloy ofsuch a metal and another metal (e.g., stainless steel) can be used.Alternatively, the positive electrode can 301 and the negative electrodecan 302 are preferably coated with, for example, nickel or aluminum inorder to prevent corrosion caused by the electrolyte solution. Thepositive electrode can 301 and the negative electrode can 302 areelectrically connected to the positive electrode 304 and the negativeelectrode 307, respectively.

The negative electrode 307, the positive electrode 304, the separator310 a, and the separator 310 b are immersed in the electrolyte solution.Then, as illustrated in FIG. 5B, the positive electrode can 301, thepositive electrode 304, the separator 310 a, the separator 310 b. thenegative electrode 307, and the negative electrode can 302 are stackedin this order with the positive electrode can 301 positioned at thebottom, and the positive electrode can 301 and the negative electrodecan 302 are subjected to pressure bonding with the gasket 303 interposedtherebetween. In such a manner, the coin-type secondary battery 300 isfabricated.

Here, a current flow in charging a secondary battery is described withreference to FIG. 5C. When a secondary battery using lithium is regardedas a closed circuit, lithium ions move and a current flows in the samedirection. Note that in the secondary battery using lithium, an anodeand a cathode change places in charge and discharge, and an oxidationreaction and a reduction reaction occur on the corresponding sides;hence, an electrode with a high redox potential is called a positiveelectrode and an electrode with a low redox potential is called anegative electrode. For this reason, in this specification, the positiveelectrode is referred to as a “positive electrode” and the negativeelectrode is referred to as a “negative electrode” in all the easeswhere charge is performed, discharge is performed, a reverse pulsecurrent is supplied, and a charging current is supplied. The use of theterms “anode” and “cathode” related to an oxidation reaction and areduction reaction might cause confusion because the anode and thecathode change places at the time of charging and discharging. Thus, theterms “amide” and “cathode” are not used in this specification. If theterm “anode” or “cathode” is used, whether it is at the time of chargingor discharging is noted and whether it corresponds to a positiveelectrode or a negative electrode is also noted.

Two terminals in FIG. 5C are connected to a charger, and a secondarybattery 400 is charged. As the charge of the secondary battery 400proceeds, a potential difference between electrodes increases. Thepositive direction in FIG. 5C is the direction in which a current flowsfrom the one terminal outside the secondary battery 400 to a positiveelectrode 402, flows from the positive electrode 402 to a negativeelectrode 404 through a separator 403 a and a separator 403 b in thesecondary buttery 400, and flows from the negative electrode 404 to theother terminal outside the secondary battery 400. In other words, acurrent flows in the direction of a flow of a charging current.

(Secondary Battery with Stacked-Layer Structure)

Next, an example of a secondary battery with a stacked-layer structurewill be described with reference to FIG. 6.

A secondary battery 500 with a stacked-layer structure illustrated inFIG. 6 includes a positive electrode 503 including a positive electrodecurrent collector 501 and a positive electrode active material layer502, a negative electrode 506 including a negative electrode currentcollector 504 and a negative electrode active material layer 505, aseparator 507 a, a separator 507 b, an electrolyte solution 508, and anexterior body 509. The separator 507 a and the separator 507 b areprovided between the positive electrode 503 and the negative electrode506 in the exterior body 509. The exterior body 509 is filled with theelectrolyte solution 508. Note that separators may have a three-layerstructure.

In the secondary battery 500 having a stacked-layer structureillustrated in FIG. 6, the positive electrode current collector 501 andthe negative electrode current collector 504 also function as terminalsfor electrical contact with an external portion. For this reason, eachof the positive electrode current collector 501 and the negativeelectrode current collector 504 is arranged so that part of the positiveelectrode current collector 501 and part of the negative electrodecurrent collector 504 are exposed outside the exterior body 500.

As the exterior body 500 in the secondary battery 500 having astacked-layer structure, a stacked film having a three-layer structurecan be used, for example. In the three-layer structure, a highlyflexible metal thin film of, for example, aluminum, stainless steel,copper, and nickel is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, and polyamide, andan insulating synthetic resin film of, for example, a polyamide-basedresin and a polyester-based resin is provided as the outer surface ofthe exterior body over the metal thin film. With such a three-layerstructure, permeation of an electrolyte solution and a gas can beblocked and an insulating property and resistance to the electrolytesolution can be provided.

(Cylindrical Secondary Battery)

Next, an example of a cylindrical secondary battery is described withreference to FIGS. 7A and 7B. As illustrated in FIG. 7A, a cylindricalsecondary battery 600 includes a positive electrode cap (battery cap)601 on its top surface and a battery can (outer can) 602 on its sidesurface and bottom surface The positive electrode cap 601 and thebattery can 602 are insulated from each other with a gasket (insulatingpacking) 610.

FIG. 7B is a schematic cross-sectional view of the cylindrical secondarybattery, inside the battery can 602 having a hollow cylindrical shape, abattery element in which a strip-like positive electrode 604 and astrip-like negative electrode 606 are wound with a separator 605positioned therebetween is provided. Note that the separator 605 has atwo-layer structure or a three-layer structure. Although notillustrated, the battery element is wound around a center pin. One endof the battery can 602 is close and the other end thereof is open. Forthe battery can 602, a metal having corrosion resistance to anelectrolyte solution, such as nickel, aluminum, or titanium, an alloy ofsuch a metal, or an alloy of such a metal and another metal (e.g.,stainless steel) can be used. Alternatively, the battery can 602 ispreferably covered with nickel, aluminum, or the like in order toprevent corrosion caused by the electrolyte solution. Inside the batterycan 602, the battery element in which the positive electrode, thenegative electrode, and the separator are wound is positioned between apair of insulating plates 608 and 609 which face each other. Further, anelectrolyte solution (not illustrated) is injected inside the batterycan 602 provided with the battery element. As the electrolyte solution,an electrolyte solution that is similar to those of the above coin-typosecondary battery and the secondary battery having a stacked-layerstructure can be used.

The strip-like positive electrode 604 and the strip-like negativeelectrode 606 can be formed in a manner similar to that of the positiveelectrode and the negative electrode of the coin-type secondary batterydescribed above; however, the difference lies in that, electrode activematerials are termed on both sides of a current collector in eachelectrode because the positive electrode and the negative electrode ofthe cylindrical secondary battery are wound. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the strip-like positive electrode 604, and a negative electrodeterminal (negative electrode current collecting lead) 607 is connectedto the strip-like negative electrode 606. Both the positive electrodeterminal 603 and the negative electrode terminal 607 can be formed usinga metal material such as aluminum. The positive electrode terminal 603and the negative electrode terminal 607 are resistance-welded to asafety valve mechanism 612 and the bottom of the battery can 602,respectively. The safety valve mechanism 612 is electrically connectedto the positive electrode cap 601 through a positive temperaturecoefficient (PTC) element 611. The safety valve mechanism 612 cuts offelectrical connection between the positive electrode cap 601 and thestrip-like positive electrode 604 when the internal pressure of thebattery exceeds a predetermined threshold value. The PTC element 611 isa heat sensitive resistor whose resistance increases as temperaturerises, and controls the amount of current by increase in resistance toprevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic or the like can be used for the PTCelement.

Note that in this embodiment, the coin-type secondary battery, thesecondary battery having a stacked-layer structure, and the cylindricalsecondary battery are given as examples of the secondary battery;however, any of secondary batteries with a variety of shapes, such as asealed secondary battery and a rectangular secondary battery, can beused. Further, a structure in which a plurality of positive electrodes,a plurality of negative electrodes, and a plurality of separators arestacked or rolled may be employed.

The coin-type secondary battery 300, the secondary battery 500, and thecylindrical secondary battery 600 described in this embodiment each havethe separator including a plurality of layers having differentcharacteristics. Therefore, the characteristics of the separator of thesecondary battery can be determined variously.

Note that this embodiment can be implemented by being combined with anyof other embodiments as appropriate.

Embodiment 4

In this embodiment, examples of a structure of a power storage deviceare described with reference to FIGS. 8A and 8B, FIGS. 9A1, 9A2, 9B1 and9B2, FIGS. 10A and 10B, FIGS. 11A and 11B, and FIG. 12.

FIGS. 8A and 8B show external views of a power storage device. The powerstorage device includes a circuit board 900 and a power storage unit913. A label 910 is attached to the power storage unit 913. Asillustrated in FIG. 8B, the power storage system further includes aterminal 951, a terminal 952, and an antenna 914 and an antenna 915which are provided behind the label 910.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. Note that the shape of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape, for example.Furthermore, a planar antenna, an aperture antenna, a traveling-waveantenna, an fill antenna, a magnetic-field antenna, a dielectricantenna, or the like may be used. Alternatively, the antenna 914 or theantenna 915 may be a flat-plate conductor. The flat-plate conductor canserve as one of conductors for electric field coupling that is theantenna 914 or the antenna 915 may serve as one of two conductors of acapacitor. Thus, electric power can be transmitted and received not onlyby an electromagnetic field or a magnetic field but also by an electricfield.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage device includes a layer 916 between the power storageunit 913 and the antennas 914 and 915. The layer 916 has a function ofblocking an electromagnetic field from the power storage unit 913, forexample. As the layer 916. for example, a magnetic body can be used.

Note that the structure of the power storage device is not limited tothat in FIGS. 8A and 8B.

For example, as illustrated in FIGS. 9A1 and 9A2, two opposite sides ofthe power storage unit 913 in FIGS. 8A and 8B may be provided with therespective antennas. FIG. 9A1 is an external view showing one of theopposite sides, and FIG. 9A2 is an external view showing the other ofthe opposite sides. Note that for the same portions as the power storagedevice in FIGS. 8A and 8B, description on the power storage device inFIGS. 8A and 8B can be referred to as appropriate.

As illustrated in FIG. 9A1, the antenna 914 is provided on one of theopposite sides of the power storage unit 913 with the layer 916 providedtherebetween, and as illustrated in FIG. 9A2, the antenna 915 isprovided on the other of the opposite sides of the power storage unit913 with a layer 917 provided therebetween. The layer 917 has a functionof blocking an electromagnetic field from the power storage unit 913,for example. As the layer 917, for example, a magnetic body can be used.

With the above structure, both the antenna 914 and the antenna 915 canbe increased in size.

Alternatively, as illustrated in FIGS. 9B1 and 9B2, two opposite sidesof the power storage unit 913 in FIGS. 8A and 8B may be provided withdifferent types of antennas FIG. 9B1 is an external view slowing one ofthe opposite sides, and FIG. 9B2 is an external view showing the otherof the opposite sides. Note that for the same portions as the powerstorage device in FIGS. 8A and 8B, description on the power storagedevice in FIGS. 8A and 8B can be referred to as appropriate.

As illustrated in FIG. 9B1, the antennas 914 and 915 are provided on oneof the opposite sides of the power storage unit 913 with the layer 916provided therebetween, and as illustrated in FIG. 9A2, an antenna 918 isprovided on the other of the opposite sides of the power storage unit913 with the layer 917 provided therebetween. The antenna 918 has afunction of performing data communication with an external device, forexample. An antenna with a shape that can be applied to the antennas 914and 915 can be used as the antenna 918, for example. As an example of amethod for communication between the power storage device and anexternal device via the antenna 918, a response method such as NFC canbe given.

Alternatively, as illustrated in FIG 10A, the power storage unit 913 inFIGS. 8A and 8B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is not necessary to provide the label 910 in a portion where thedisplay device 920 is provided. Note that for the Name portions as thepower storage system in FIGS. 8A and 8B, description on the powerstorage device in FIGS. 8A and 83 can be referred to us appropriate.

The display dev ice 920 can display, for example, an image showingwhether charging is being carried out or an image showing the amount ofstored power. As the display device 920, electronic paper, a liquidcrystal display device, an electroluminescent (EL) display device, orthe like can be used. For example, the power consumption of the displaydevice 920 can be reduced when electronic paper is used.

Alternatively, as illustrated in FIG. 10B, the power storage unit 913 inFIGS. 8A and 88 may be provided with a sensor 921. The sensor 921 iselectrically connected to the terminal 911 via a terminal 922. Note thatthe sensor 921 may be provided behind the label 910. Note that for thesame portions as the power storage device in FIGS. 8A and 8B,description on the power storage device in FIGS. 8A and 8B can bereferred to as appropriate.

For the sensor 921, any of a variety of sensors can be used. With thesensor 921, for example, data on an environment (e.g., temperature)where the power storage device is placed can be determined and stored ina memory inside the circuit 912.

Furthermore, structural examples of the power storage unit 913 aredescribed with reference to FIGS. 11A and 11B and FIG. 12.

In the power storage unit 913 illustrated in FIG. 11A, a wound body 950having the terminals 951 and 952 is provided in a housing 930. The woundbody 950 is soaked in an electrolyte solution inside the housing 930.The terminal 952 is in contact with the housing 930. An insulator or thelike prevents contact between the terminal 951 and the housing 930. Notethat FIG. 11A illustrates the housing 930 divided into two pieces forconvenience; however, in the actual structure, the wound body 950 iscovered with the housing 930 and the terminals 951 and 952 extend to theoutside of the housing 930. For the housing 930, a metal material (e.g.,aluminum) or a resin material can be used.

Note that the housing 930 illustrated in FIG. HA may be formed using aplurality of materials. For example, in the power storage unit 913 inFIG. 11B, a housing 930 a and a housing 930 b are attached to each otherand the wound body 950 is provided in a region surrounded by the housing930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, an electric field canbe prevented from being blocked by the power storage unit 913. When anelectric field is not significantly blocked by the housing 930 a, anantenna such as the antenna 914 or the antenna 915 may be providedinside the housing 930. For the housing 930 b, a metal material can beused, for example.

FIG. 12 shows a structure of the wound body 950. The wound body 950includes a negative electrode 931, a positive electrode 932, and aseparator 933. Note that the separator 933 has a two-layer structure ora three-layer structure. The wound body 950 is obtained by winding asheet of a stack in which the negative electrode 931 overlaps with thepositive electrode 932 with the separator 933 provided therebetween.Note that a plurality of sheets each including the negative electrode931, the positive electrode 932, and the separator 933 may be stacked.

The negative electrode 931 is connected to the terminal 911 in FIGS. 8Aand SB via one of the terminals 951 and 952. The positive electrode 932is connected to the terminal 911 in FIGS. 8A and 8B via the other of theterminals 951 and 952.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 5

In this embodiment, examples of electronic devices including any of thesecondary batteries illustrated in the above embodiments are describedwith reference to FIGS. 13A to 13F and FIGS. 14A to 14C.

Examples of electronic devices including secondary batteries are camerassuch as digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cellular phones or portable telephonedevices), portable game consoles, portable information terminals, andaudio reproducing devices. FIGS. 13A to 13F und FIGS. 14A to 14Cillustrate specific examples of these electronic devices.

FIG. 13A illustrates an example of a mobile phone. A mobile phone 800 isprovided with a display portion 802 incorporated in a housing 801, anoperation button 803, a speaker 805, a microphone 806, and the like. Theuse of a secondary battery 804 of one embodiment of the presentinvention in the mobile phone 800 results in weight reduction.

When the display portion 802 of the mobile phone 800 illustrated in FIG.13A is touched with a finger or the like, data can be input into themobile phone 800. Users can make a call or text messaging by touchingthe display portion 802 with their fingers or the like.

There are mainly three screen modes for the display portion 802. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as characters. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion802 so that text displayed on a screen can be inputted.

When a sensing device including a sensor such as a gyroscope and anacceleration sensor for detecting inclination is provided in the mobilephone 800. display on the screen of the display portion 802 can beautomatically changed in direction by determining the orientation of themobile phone 800 (whether the mobile phone 800 is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 802 orOperating the operation button 803 of the housing 801. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 802. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 802 is detected and the input by touch on thedisplay portion 802 is not performed for a certain period, the screennode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 802 can function as an image sensor. For example, animage of a palm print, a fingerprint, or the like is taken with thedisplay portion 802 touched with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or a sensing light source that emits near-infrared light inthe display portion, an image of a finger vein, a palm vein, and thelike can be taken.

FIG. 13B illustrates the mobile phone 800 that is bent. When the wholemobile phone 800 is bent by the external force, the secondary battery804 included in the mobile phone 800 is also bent. FIG. 13C illustratesthe bent secondary battery 804. The secondary battery 804 is a secondarybattery having a stacked-layer structure.

FIG. 14A illustrates a smart watch. The smart watch can include ahousing 702, a display panel 704, operation buttons 711 and 712, aconnection terminal 713, a band 721, a clasp 722, and the like. The useof the secondary battery of one embodiment of the present invention inthe smart watch results in weight reduction. The coin-type secondarybattery of one embodiment of the present invention which is described inEmbodiment 3 may be included in the housing 702 (FIG. 14B). A flexiblesecondary battery 740 of one embodiment of the present invention may beincluded in the band 721 of the secondary battery. The flexiblesecondary battery 740 can have a band shape, in which case the flexiblesecondary battery 740 can be attachable to and detachable from thehousing 702. Electric power can be supplied to the housing 702 through apositive electrode terminal 741 and a negative electrode terminal 742(FIG. 14C).

The display panel 704 mounted in the housing 702 serving as a bezelincludes a non-rectangular display region. The display panel 704 candisplay an icon 705 indicating time and another icon 706.

The smart watch illustrated in FIGS. 14A to 14C can have a variety offunctions, for example, a function of displaying a variety of data(e.g., a still image, a moving image, and a text image) on a displayportion, a touch panel function, a function of displaying a calendar,date, time, and the like, a function of controlling processing with avariety of software (programs), a wireless communication function, afunction of being connected to a variety of computer networks with awireless communication function, a function of transmitting andreceiving a variety of data with a wireless communication function, anda function of reading program or data stored in a recording medium anddisplaying the program or data on a display portion.

The housing 702 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and so on.

FIG. 13D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a secondary battery 7104. FIG. 13Eillustrates the bent secondary battery 7104.

FIG. 13F illustrates an example of an armband display device. An armbanddisplay device 7200 includes a housing 7201 and a display portion 7202.Although not Shown, a flexible secondary battery is included in thearmband display device 7200. The flexible secondary battery changes inshape in accordance with change in the shape of the armband displaydevice 7200.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

This application is based on Japanese Patent Application serial no.2013-236697 filed with Japan Patent Office on Nov. 15, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A nonaqueous secondary battery comprising: apositive electrode; a first separator over the positive electrode; asecond separator over and in contact with the first separator; and anegative electrode over the second separator; wherein the firstseparator includes a first pore, wherein the second separator includes asecond pore, and wherein a size of the second pore is smaller than asize of the first pore.
 2. The nonaqueous secondary battery according toclaim 1, wherein a thickness of the first separator is different from athickness of the second separator.
 3. The nonaqueous secondary batteryaccording to claim 1, wherein a thickness of the second separator issmaller than a thickness of the first separator.
 4. The nonaqueoussecondary battery according to claim 1, wherein the negative electrodecomprises a first surface and a second surface opposite to the firstsurface, and wherein the second separator is in contact with the firstsurface and the second surface.
 5. The nonaqueous secondary batteryaccording to claim 1, wherein the positive electrode comprises apositive electrode active material layer, wherein the positive electrodeactive material layer comprises a positive electrode active material,and wherein the positive electrode active material comprises alithium-containing material having any one of an olivine crystalstructure, a layered rock-salt crystal structure, and a spinel crystalstructure.
 6. The nonaqueous secondary battery according to claim 5,wherein the positive electrode active material layer further comprises aconductive additive including any one of acetylene black, graphiteparticles, carbon nanotubes, graphene, and fullerene.
 7. The nonaqueoussecondary battery according to claim 1, wherein the negative electrodecomprises a negative electrode active material layer, wherein thenegative electrode active material layer comprises a negative electrodeactive material, and wherein the negative electrode active materialcomprises any one of graphite, graphitizing carbon, non-graphitizingcarbon, a carbon nanotube, graphene, carbon black, and a materialincluding at least one of Ga, Si, Al, Ge, Sn, Pb, Sb, Bi, Ag, Zn, Cd andIn.
 8. A nonaqueous secondary battery comprising: a positive electrode;a first separator over the positive electrode; a second separator overand in contact with the first separator; and a negative electrode overthe second separator; wherein the first separator includes first pores,wherein the second separator includes second pores, and wherein aproportion of a volume of the second pores in the second separator islower than a proportion of a volume of the first pores in the firstseparator.
 9. The nonaqueous secondary battery according to claim 8,wherein a thickness of the first separator is different from a thicknessof the second separator.
 10. The nonaqueous secondary battery accordingto claim 8, wherein a thickness of the second separator is smaller thana thickness of the first separator.
 11. The nonaqueous secondary batteryaccording to claim 8, wherein the negative electrode comprises a firstsurface and a second surface opposite to the first surface, and whereinthe second separator is in contact with the first surface and the secondsurface.
 12. The nonaqueous secondary battery according to claim 8,wherein the positive electrode comprises a positive electrode activematerial layer, wherein the positive electrode active material layercomprises a positive electrode active material, and wherein the positiveelectrode active material comprises a lithium-containing material havingany one of an olivine crystal structure, a layered rock-salt crystalstructure, and a spinel crystal structure.
 13. The nonaqueous secondarybattery according to claim 12, wherein the positive electrode activematerial layer further comprises a conductive additive including any oneof acetylene black, graphite particles, carbon nanotubes, graphene, andfullerene,
 14. The nonaqueous secondary battery according to claim 8,wherein the negative electrode comprises a negative electrode activematerial layer, wherein the negative electrode active material layercomprises a negative electrode active material, and wherein the negativeelectrode active material comprises any one of graphite, graphitizingcarbon, non-graphitizing carbon, a carbon nanotube, graphene, carbonblack, and a material including at least one of Ga, Si, Al, Ge, Sn, Pb,Sb, Bi, Ag, Zn, Cd and In.
 15. A nonaqueous secondary batterycomprising: a positive electrode; a first separator over the positiveelectrode; a second separator over and in contact with the firstseparator; a third separator over and in contact with the secondseparator; and a negative electrode over the third separator, whereinthe first separator includes a first pore, wherein the second separatorincludes a second pore, wherein the third separator includes a thirdpore, wherein a size of the second pore is different from a size of thefirst pore and a size of the third pore.
 16. The nonaqueous secondarybattery according to claim 15, wherein the first separator and the thirdseparator each include a material, wherein the second separatorcomprises a material which is different from the material including thefirst separator and the third separator.
 17. The nonaqueous secondarybattery according to claim 15, wherein a thickness of the secondseparator is larger than a thickness of the first separator and athickness of the third separator.
 18. The nonaqueous secondary batteryaccording to claim 15, Wherein the positive electrode comprises a firstsurface and a second surface opposite to the first surface, Wherein thefirst separator is in contact with the first surface and the secondsurface, wherein the negative electrode comprises a third surface and afourth surface opposite to the third surface, and wherein the thirdseparator is in contact with the third surface and the fourth surface.19. The nonaqueous secondary battery according to claim 15, wherein thepositive electrode comprises a positive electrode active material layer,wherein the positive electrode active material layer comprises apositive electrode active material, and wherein the positive electrodeactive material comprises a lithium-containing material having any oneof an olivine crystal structure, a layered rock-salt crystal structure,and a spinel crystal structure.
 20. The nonaqueous secondary batteryaccording to claim 19, wherein the positive electrode active materiallayer further comprises a conductive additive including any one ofacetylene black, graphite particles, carbon nanotubes, graphene, andfullerene.
 21. The nonaqueous secondary battery according to claim 8,wherein the negative electrode comprises a negative electrode activematerial layer, wherein the negative electrode active material layercomprises a negative electrode active material, and wherein the negativeelectrode active material comprises any one of graphite, graphitizingcarbon, non-graphitizing carbon, a carbon nanotube, graphene, carbonblack, and a material including at least one of Ga, Si, Al, Ge, Sn, Pb,Sb, Bi, Ag, Zn, Cd and In.